In a conventional off-line switching mode power supply, in order to regulate output voltage and current of the power supply, output voltage and current sensors, such as shunt regulator, current sense resistor and opto-coupler to deliver feedback signal with hazard voltage isolation, are needed on secondary side. In further detail, FIG. 1 shows a conventional flyback converter 100, in which a transformer T1 has a primary winding 102 coupled between a voltage source Vin and a power switch S1, a secondary winding 104 coupled between a rectifier diode Do and a ground terminal GND on secondary side, and an auxiliary winding 106 coupled between a ground terminal GND and a rectifier diode Daux on primary side, and a controller 108 provides a control signal Vgs to switch the power switch S1 to convert the input voltage Vin to an output voltage Vo for a load RL. By switching the power switch S1, power is delivered from the primary winding 102 to the secondary winding 104 and thus produces current that will be rectified by the rectifier diode Do and charge the output capacitor Co to produce the output voltage Vo on the power output. On the other hand, the auxiliary winding 106 on the primary side produces current that will be rectified by the diode Daux and charges a capacitor Caux to produce a direct current (DC) voltage on the capacitor Caux, for serving as a power source for the controller 108. FIG. 2 is a diagram to show the output characteristic of the flyback converter 100, which has constant voltage mode and constant current mode. As shown by the curve, the flyback converter 100 operates at constant output voltage Vc when the output load RL is less than a threshold, and once the output load RL increases, the load current Io would be limited at Ic and the output voltage Vo drops following the load impedance RL. In order to regulate the output voltage Vo and current Io, as shown in FIG. 1, current sense resistor Rcs, opto-coupler 110 and shunt regulator 112 are required on the secondary side to detect the output voltage Vo and current Io to produce a feedback signal FB for the controller 108, and these devices will increase the size and cost of the flyback converter 100.
To overcome the above problem, the output voltage Vo and current Io are alternatively extracted on the primary side of the transformer T1 or by using the auxiliary winding 106, for example, disclosed by U.S. Pat. Nos. 5,438,499, 6,480,399, 6,721,192, 6,781,357, 6,833,692, 6,862,194, 6,967,472 and 6,977,824. However, due to the forward voltage of the rectifier diode Do and the leakage inductance of the transformer T1, these arts could not accurately obtain the output information.